1. Field of the Invention
The present invention relates to a method and system for an optical remote measurement of the temperature of the air, particularly in front of an aircraft, in a zone not disturbed by its movement.
The temperature of the air is one of the parameters which are displayed on the instrument panel of an aircraft and which are used, for controlling and piloting the aircraft, to elaborate then display some of the parameters of the aircraft, such for example as its airspeed, the mach number, its ground speed, its altitude, its power setting.
It was then a priori advantageous to invent an optical temperature measurement eliminating the drawbacks of conventional local measurements using a thermometer, which are subject to aerodynamic disturbances which must then be corrected.
Methods are already known for measuring the air temperature based on the laser induced fluorescence of oxygen. For putting them into practice, either an ArF (argon fluorine) laser, with a wavelength of 193 nm or a KrF laser (krypton fluorine), having a wavelength of 248 nm are used for energizing hot bands, at a high vibratory level, of the Schumann-Runge band of oxygen.
According to the Applicant, these known methods have only been used in the laboratory for studying the temperature distribution in flames and hot air flows. But these methods are only applicable with low absorption, i.e. when the exponential attenuation factor of the laser beam as a function of the distance can be linearized. In this case, there is proportionality between the fluorescence intensities and the populations of the lower states of the excited transitions, which makes it possible to calculate the temperature without knowing the oxygen density. In practice, that is tantamount to limiting the measurement distance to about 10 cm. To calculate the temperature in front of an aircraft, in an undisturbed zone, namely at about 10 m in front of the aircraft, these known methods cannot be applied because of the high attenuation of the beam. The fluorescence signals would be very complex functions of the temperature, of the density and of the spectral distribution of the laser and calculation would be practically impossible.
2. Description of the Prior Art
As part of the prior art there may be mentioned the article "Laser induced fluorescence with tunable exciter lasers as a possible method for instantaneous field measurements at high pressure : checks with an atmospheric flame, Andresen et al., 15 Jan. 1988, vol 27, no. 2, Applied Optics". This document deals with flame temperature measurements by excitation of OH, O.sub.2 or H.sub.2 O by a KrF laser and relates more particularly to the excitation of two transitions of the molecule considered by means of two laser pulses offset in frequency and determination of the ratio of the fluorescence intensities, via the populations of the lower levels of the two transitions, which only depends on the temperature.
The article "Quantitative imaging of temperature fields in air using planar laser induced fluorescence of O.sub.2, Lee et al., 1987, vol 12, no. 2, Optics Letters" can also be cited. This document deals with the formation of two dimensional images of the temperature in hot air flows. It teaches focussing an untuned wide band ArF laser on the measurement volume and observing the fluorescence. Then spectral oxygen absorption lines are excited and the overall fluorescence, under low absorption conditions, may be imaged as a function of the temperature, except for a multiplier coefficient, proportional to the density. After absolute calibration, a thermal map may be formed having very good spatial resolution.
The article "Feasibility of measuring temperature and density fluctuations in air using laser induced O.sub.2 fluorescence, Massey and Lemon, May 1984, vol QE-20, no. 5, IEEE Journal of Quantum Electronics" may further be mentioned. This document deals with the measurement of temperature and density fluctuations in space or in time and teaches using an ArF laser, with an averagely improved spectral line, for exciting two adjacent spectral lines P and R of the oxygen band [X.sub.0, B.sub.4. With low absorption, the relative fluorescence variation observed may be linked to the relative temperature variation. For calculating this relative temperature variation, two laser pulses are used offset in frequency, exciting two pairs of spectral lines P(J.sub.1), R(J+2) and P(J.sub.2), R(J.sub.2 +2), respectively.
Finally, the article "Proposed single pulse two dimensional temperature and density measurements of oxygen and air, Miles et al., March 1988, vol 13, no. 3, Optics Letters" may be mentioned. This document deals with the same subject as the preceding one and teaches using an ArF laser tuned to a UV transition of oxygen and observing the Rayleigh diffusion and the fluorescence. With low absorption and at a temperature less than 500 K, the intensity of the Rayleigh backscatter is proportional to the density and is independent of the temperature and the fluorescence intensity depends on the density but also on the temperature. The ratio of the fluorescence and Rayleigh diffusion intensities only depends on the temperature and can be imaged.
The present invention proposes then first of all a method for the optical measurement of the air temperature, at a "great" distance.